The present invention relates to a control apparatus for an on-demand ink jet printing element in which printing is effected by producing from a logic signal an electrical current such as to increase the temperature of the ink disposed in a nozzle, in which a first pulse is generated to raise the temperature of the ink in the nozzle to close to the vaporization point and a second pulse is generated to create a bubble of vapour in the nozzle so as to cause a drop of ink to be expelled through the nozzle.
In a printing arrangement as disclosed in British patent specification GB No. 2 106 039, the current passes through a resistor disposed in the nozzle in the vicinity of the outlet edge. Two pulses are generated one after the other by two different voltages whereby they are of fixed duration. Upon a variation in ambient temperature therefore, the effect of the two pulses on the drop of ink also varies, so that the strength of the printing increases with ambient temperature.